Gamma ray spectroscopy well logging tools are significantly expanding the use of multiple radioactive isotope tracers in monitoring down hole operations. These well logging tools can measure the energy of gamma rays which are emitted by radioactive tracers placed in the well being logged Multiple tracer surveys may be obtained by deconvolving the gamma ray spectral data into contributions from each individual isotope as a function of depth. Multiple tracer logs can give such useful information as fluid fracture height versus propant height as well as the effectiveness of multi-stage treatments and hydraulic fracturing operations. Well cementing applications can include the locations of different types of cements and measurement of average radial cement thickness.
A potential problem which can occur in the use of multiple tracers in a gamma ray spectroscopy tracer logging system is that, due to variations in the Compton scattering and photoelectric absorption of gamma rays between the tracer source and the detector, the shape of the gamma ray spectrum can change with changes in tracer location. The change of shape of such gamma ray energy spectra due to Compton scattering and photoelectric absorption can impair a weighted least-square fitting technique which assumes as the standard spectra those taken but with the tracer in a particular geometrical region (i.e., the formation), if this region is different from the actual tracer location in the well of interest For example, if some of the tracers in a well borehole are actually located inside the casing, but the standard spectra were derived for tracers located only in the formation , the computed tracer concentration will be in error.
Studies conducted by the inventors have indicated that more accurate results in the use of multi-isotopes can be achieved wherein compensation techniques are used to correct the formation tracer concentrations computed from the observed gamma ray spectra for the occurrence of isotope contamination in the well borehole. Since the spectral shape from each tracer element is a function of both the energies of the gamma rays emitted and the location of the tracer in the downhole geometry, it is possible to develop standard tracer spectral shapes for each tracer in both the borehole and in the formation. The least squares fitting therefore can incorporate spectral components for each tracer in both the borehole and formation. The least-square solution will therefore not only determine the total tracer concentration for each isotope, but also the relative distribution of each tracer between the borehole and the formation.